1. Field of the Invention
This invention pertains to the recovery and recycling of organic acids, water and other valuable wash water constituents produced in the manufacture of terephthalic acid.
2. Background of the Invention
Technical-grade terephthalic acid is produced primarily by the liquid-phase air oxidation of p-xylene. In a typical process, an oxidation reactor is provided and solvent, air, p-xylene and catalyst are continuously fed into the reactor where oxidation takes place at elevated temperatures. Acetic acid can be used as the solvent, one or more multivalent metals, such as cobalt and manganese, can be used as the catalyst, and bromine can be used as a renewable source of free radicals. The terephthalic acid leaves the reactor in the form of a slurry and, after passing the slurry through a surge vessel which operates at a lower temperature and pressure than the reactor, the acid is recovered by centrifuging, washing and drying the acid crystals. A typical terephthalic acid manufacturing process involving liquid-phase air oxidation of p-xylene is depicted in Kirk-Othmer, Encyclopedia of Chemical Technology, Vol 17 (1982), on page 749. Other terephthalic manufacturing operations involve similar steps and reactants.
When the terephthalic acid crystals are washed, the wash water carries away uncrystallized terephthalic acid, unwanted byproducts such as p-toluic acid and benzoic acid, and catalysts from the terephthalic acid crystals. Terephthalic acid manufacturing plants generate large amounts of such wash water and environmental control standards prevent the discharging of the untreated water into natural waterways. In the past, the environmentally offensive components of the wash water have been removed by retaining the water in biological lagoons for extended periods of time, whereby microbial action reduces certain components to innocuous constituents. However, this treatment is expensive and time-consuming. Further, the biological lagoons do not always remove all of the components contained in the wash water, so that recycling of the water is not possible.
Importantly, previous treatments of the wash water have also failed to enable recovery and reuse of any components of the wash water. When the components are removed by treatment in biological lagoons, they are either destroyed by bacteria or deposited on the lagoon bottom in the form of unusable sludge.
Accordingly, there exists a need in the art for an improved process for treating wash water produced in the manufacture of terephthalic acid. The process should be capable of removing components from the wash water, such that the water can be discharged into natural waterways or, preferably, recycled for reuse in the manufacture of additional terephthalic acid. Preferably, the process should enable the recovery of valuable components of the wash water, such that the components can be reused in the manufacture of additional terephthalic acid. In an especially preferred embodiment, the process should be practiced in an apparatus, the regeneration of which would be accomplished by regenerating substances that could also be recovered and reused in the acid manufacturing process. The process should be efficient and economical and thus would advantageously eliminate the need for large areas presently required for prior art biological lagoons. These objectives are satisfied by the present invention. Other objectives of the invention will become apparent from the following.